1. Field of the Invention
This invention relates to an improved reflecting-refracting high speed objective lens in which all optical elements are assembled in two members.
2. Description of the Prior Art
The reflecting objective lens, as initially used in astronomy, is distinguished by its ideally achromatic image which is exceptionally useful with large focal lengths. A further advantage over refracting lenses is given by the reduced length which enhanced the use of reflecting telephoto lenses in photography.
Recently reflecting lenses have proven very useful for electronic infrared- and night-vision systems featuring high speed--high contrast objectives with reduced image angle. Reflecting-refracting or catadioptric lenses were designed in order to use only spherical mirrors, to extend the field of view and to reduce image errors by introducing ordinary refracting lenses for correction purposes. The thus created Maksutov-lens nevertheless exhibits a large zonal error of spherical aberration.
Another similar type, the Mandler-lens, having a two member front part (it likewise presents a large zonal error of spheric aberration) is disclosed e.g. in F. I. Havlicek, L. Canzek "Zum Korrigieren von Spiegelobjektiven", Deutsche Geodatische Kommission bei der Bayerischen Akademie der Wissenschaften, Reihe A, Heft 42, Munchen 1963.
A further improvement was brought about by using a one member front lens, the silvered central part of which constitutes the second mirror, while the first or main mirror has a central bore as disclosed by Swiss Pat. No. 542,454. These known objective lenses nevertheless suffer from at least one of the following short-comings:
(a) A central bore is applied to the main mirror involving a high cost, machining of a ring-shaped lens is difficult, and such a lens has a reduced mechanical stability as compared to a solid lens.
(b) The front lens, the main mirror and the field lens system are rather sensitive to centering. Without exact centering troublesome comatic and astigmatic defects are unavoidable.
(c) Mounting of the field lenses within the objective requires a first precise mount, another mount is necessary for securing the field lens system within the objective housing. This multi-mounting method involves centering errors which in turn cause coma and astigmatism.